Same problem here. After updating to C66 , when I try to create a new account, the screen just hangs up. Tried to upgrade to Edge, just sits there. Tried to down grade to Release, same thing. This is nuts !

Looks like when you change an accounts IP, that is what is locking up cpanel
Just did it again / killed the change IP process and killall -9 safeapacherestart , now back to normal.
Add tried to up to 10.6.0 (build 85) - EDGE but stays at E80